Inclusion of radioactive waste in glass molds could involve more risks than previously assumed
Simulation of structural defects in silicate glass after firing with neutrons
Los Angeles (USA) - For highly radioactive wastes, there are glazing processes that allow the substances to be enclosed in resistant borosilicate glass molds. The disordered glass-like structure of the material offers the advantage of high stability against heat and acids and should be self-healing in the case of radioactive radiation. Scientists at the University of California in Los Angeles now doubt this process. They simulated the behavior of silicate glasses under neutron bombardment. Their results published in the journal "Journal of Chemical Physics" show that the structural changes during irradiation are more prone to a previously unknown disorder as in liquids. If this behavior can also be confirmed experimentally, the stability of glass molds would have to be reassessed.
"The atomic structure of the irradiated material is more like a liquid than a glass," says Mathieu Bauchy from the UCLA working group for amorphous and inorganic solids. Together with his colleagues, he determined the structure of amorphous silicon dioxide with molecular dynamics simulations. The basis for their calculations was a supercell of 8100 atoms - 5400 oxygen and 2700 silicon atoms. With these simulations they determined the probabilities for the positions of the oxygen and silicon atoms. From these data, it was able to conclude on marked structures and defects in the disordered silicate structure.
For the material changes by radioactive radiation they proceeded from a shot with fast neutrons. These neutrons, like binary balls, impacted the atoms in the silicate compound and led to a change in the silicate structure by means of a chain reaction. For comparison, they started a second simulation for shock-like solidified silicate glass with a melt of more than 4,000 degrees, which was cooled very quickly to room temperature.
Both simulations showed a very similar structure at first sight over the entire supercell. The density of the material was almost identical. However, significant differences between the solidified melt and the silicate after neutron bombardment were evident in the short-range order of the atoms at distances of about one tenth nanometer. The silicate, which was virtually neutron-coated, exhibited significantly more defects on this scale, for example in the form of ring structures with six to ten atoms.
This simulation shows that, despite detailed structural analyzes of silicate glasses, a damaging influence of neutron bombardment on the internal structure has been underestimated. According to Bauchy alone, the distinctive differences between the two simulated silicate structures suggests that these glasses could be significantly altered by radioactive radiation. The hitherto meager understanding of the influence of fast neutrons on the structure of silicates, the researchers consider a serious risk for the safe containment of radioactive substances in glass molds.
Now the researchers want to extend their simulations also to cement materials, which are used in nuclear power stations. Their goal is to establish new models for the long-term stability of materials under radioactive radiation. In parallel, concrete structural analyzes of material samples would also be necessary in order to experimentally verify the results of the simulations.